Many in the field of coated steel products have attempted to provide a pre-coated strip to high speed metal forming operations resulting in finished or semi-finished formed products with functional coating layers which may or may not require further treatment or processing to provide final coating layer performance characteristics. The oldest and probably the best known method which has been in use for many years to provide such coated strips for various applications involves the use of a coil coating layer process in which a liquid paint or coating layer is applied to a moving strip by transfer of the liquid from a reservoir to a roll and finally to the surface of the metal strip. After a coating layer is applied, the coated strip is transported through an oven in which the coating layer is cured, a process whereby the coating layer is polymerized or cross-linked to give final coating layer attributes while removing carrier solvents which were part of the initial coating layer system.
Polymers used in these applications typically have been restricted to thermoset resins which have relatively narrow fields of application. Consequently, depending on the specific intended application, a number of such resins may be required. These existing processes are extremely environmentally unfriendly and generate large amounts of volatile organic compounds from the applied coating layers during the cure cycle. Accordingly, facilities using these processes require significant hazardous waste handling capabilities.
Since the early 1970's, a number of processes involving lamination have been proposed as possible alternative approaches for producing coated metal strip which processes eliminate or reduce many of the environmental factors associated with coil coating layer. One of the first known processes is described in U.S. Pat. No. 3,679,513 (1972) which describes a lamination process for applying a polyethylene film to a metal strip by heating the metal strip to a temperature above the melting point of polyethylene film and passing this heated strip along with the polyethylene film through a set of nip rolls whereby the plastic film interacts with the metal surface to effect good adhesion. From this point, the coated metal strip passes through a “post heating zone” for development of final coating layer attributes.
In 1985, in U.S. Pat. No. 4,517,255, a process was described for the production of metal sheet with a polyester resin film laminate again using a pre-heated strip followed by application of the polyester film by nip rolls with no further post-treatment of the coated metal strip. The thermoplastic polyester films used were either crystalline or oriented depending on the intended application of the resulting product.
During the 1990's, a number of U.S. Patents were issued to Heyes et. al. involving the production of polymer metallic strip laminates (See U.S. Pat. No. 5,059,460 (1991), U.S. Pat. No. 5,093,208 (1992), U.S. Pat. No. 5,238,517 (1993), U.S. Pat. No. 4,957,820 (1990), U.S. Pat. No. 4,980,210 (1990), U.S. Pat. No. 5,149,389 (1992), and U.S. Pat. No. 5,318,648 (1994)). All of these patents are based on the use of pre-heated metal strip to which is applied various thermoplastic polymers and polymer combinations using nip rolls followed by post-heating and water quench. Processing differences include various pre-heat temperatures, post-heat temperatures, delay times before quench, and quench temperatures. Various thermoplastic polymers and polymer combinations are claimed by Heyes et.al. along with coating layer functionality and attributes resulting from the various means of processing.
Similar patents were also granted to Tanaka et. al. (U.S. Pat. No. 5,330,605 (1994)), Newcomb et. al. (U.S. Pat. No. 5,679,200 (1997)), Sakamoto et. al. (U.S. Pat. No. 6,017,599 (2000)), Yasunaka et. al. (U.S. Pat. No. 6,080,260 (2000)), and Kaguma et. al. (U.S. Pat. No. 6,164,358 (2000)), which all describe similar processing technology as those described above with unique variations of process parameters to yield coated products with specific performance characteristics.
All of the lamination processes described thus far have involved lamination of pre-cast thermoplastic films, produced in a first and separate step using well-known extrusion film casting technology, before application to a hot metal strip. In 1990, Gregory (U.S. Pat. No. 4,941,935) described a process for coating layer aluminum foil or paper/aluminum foil laminate using a direct co-extrusion process. Coating layer co-extruded using this technique consisted of a layer of olefin terpolymer and a layer of polypropylene, the olefin terpolymer being adjacent to the substrate and the polypropylene being adjacent to the olefin terpolymer.
Subsequently, U.S. Pat. No. 5,407,702 (1995) disclosed a direct extrusion coating layer process for coating layer both sides of a metal strip, such as aluminum alloy strip, which is moved through a pre-conditioner, two extrusion dies, a post heater, and finally a cooling system. Both sides of the strip are coated with thin coating layers of polyester material. The resulting coated strip is useful for containers, such as cans and can ends, as well as for automobiles, appliances, aerospace, construction, and electrical devices. This patent was followed by U.S. Pat. No. 5,919,517 (1999) in which Levendusky et. al. disclosed a method and apparatus for coating layer both sides of a metal strip, such as aluminum alloy strip. The teachings in this patent are essentially identical to those in U.S. Pat. No. 5,407,702 except no post-heating or post-treating is required after extrusion application of the coating layer(s).
Finally, Schmid et.al. disclose in U.S. Pat. No. 5,942,285 (1999) a method for extrusion coating layer a metal substrate, solid extrusion coating layer compositions for metal substrates, and a metal article. The extrusion coating layer composition is a thermoplastic material and comprises: (a) a polyester having an average molecular weight of about 10,000 to about 35,000, and (b) a modifying resin, such as an epoxy resin having an epoxy equivalent weight of about 500 to about 15,000. The extrusion coating layer composition is applied to a metal substrate in an extrusion process to provide a composition film having a thickness of about 1 to about 40 microns.
Although many of the prior art methods provide coated metal strip or sheet, many of them require the formation of a film for lamination before application to the metal substrate. Further, many of these methods which provide direct coating layers onto a metal substrate using an extrusion coating layer process do not provide a continuous process for strip coating layer which operates from uncoiler to coiler. Others do not have the features or characteristics of ultra rapid deposition of the coating layer and in particular do not possess an edge containment system which allows rapid deposition of a broad range of resins. Further, many do not provide an applied coating layer with chemical and physical properties comparable to those of laminate coated products and in particular do not describe a process in which a priming step is used as part of that process to improve adherence of such coating layer.
Accordingly, it is an object of the present invention to provide a method for the continuous coating layer of metal strip, particularly of various types of metal strip, with a thermoplastic resin coating layer in which many different polymeric resins can be applied at ultra high speeds on a metal substrate. It is the further object of the present invention to provide a method for coating layer a metal strip from uncoiler to coiler at relatively high speed with uniformity of coating layer thickness and consistency of texture and color. It is also an object of the invention to provide for a method of continuous coating of a metal strip which employs an edge containment system. Finally, it is the object of the present invention to provide a metal composite comprising a metal strip having a primer coating layer along with a tightly adhered uniform thermoplastic coating layer with defined crystallinity after post-treatment using a controlled quench process to achieve required chemical and physical properties of the resulting coating layer. Other advantages of this invention will become apparent with the detailed description herein.